Obstructive Sleep Apnea (OSA) and other dangerous sleep-disordered breathing (SDB) conditions affect thousands worldwide. For example, patients suffering from SDB may exhibit breathing conditions commonly referred to as apneas and/or hypopneas. Apneas are periods of time during which breathing stops or is markedly reduced. Apneas usually are measured during sleep over a given time period. Hypopneas are decreases in breathing that are not as severe as apneas. Like apneas, hypopneas usually disrupt the level of sleep. More particularly, hypopneas are a result of decreased airflow by 30-50% for at least 10 seconds, and are sometimes associated with a SpO2 desaturation of 4% or greater. Hypoventilation may be associated with non-obstructive alveolar etiology, neuromuscle disease, and chest wall deformity; it also can be related to obesity and congenital central diseases. Typically, patients suspected of suffering from an SDB register with a certified sleep laboratory where sleep technicians fit patients with numerous data collectors and monitor their sleep activity over a given period.
Information relating to apneas and/or hypopneas can be monitored and processed. For example, an apnea index (AI) can estimate the severity of apnea. AI is calculated by dividing the number of apneas by the number of hours of sleep. The greater the AI, the more severe the apnea. Similarly, a hypopnea index (HI) can be calculated by dividing the number of hypopneas by the number of hours of sleep. Clinical examiners also can calculate and track an Apnea-Hypopnea Index (AHI). AHI is an index of severity that combines apneas and hypopneas. Combining apnea and hypopnea information gives an overall severity of sleep apnea including sleep disruptions and desaturations (e.g. a low level of oxygen in the blood). AHI, like AI and HI, is calculated by dividing the number of apneas and hypopneas by the number of hours of sleep. The following table provides exemplary AHI ranges and their corresponding levels of severity. It will be appreciated that the following tables are provided by way of example and without limitation, and that it may be possible to classify the level of severity using other AHI ranges.
AHILevel of Severity0-5Normal 5-15Mild apnea15-30Moderate apnea30 or moreSevere
Numerous techniques have emerged for treating and/or normalizing apneas, including, for example, introducing positive airway pressure to the patient. This technique pneumatically splints the airway open and reduces the number of apneic events. One example of such techniques makes use of Continuous Positive Airway Pressure (CPAP) devices, which continuously provide pressurized air or other breathable gas to the entrance of a patient's airways via a patient interface (e.g. a mask) at a pressure elevated above atmospheric pressure, typically in the range 3-20 cm H2O.
FIG. 1 is a positive airway pressure generator in the prior art. In FIG. 1, PAP device 10 delivers a supply of pressurized, breathable gas to patient 12. The pressurized breathable gas is provided from PAP device 10 to patient 12 via flexible tube 14. Flexible tube 14 terminates with mask 16, which may, for example, be fitted for the nose patient 12. An operator, sleep clinician, or similar person may use controls 18 to specify treatment parameters such as, for example, pressure thresholds, treatment duration, type of information to monitor, etc. Processor 20 interprets these commands and instructs motor 22 to provide the supply of pressurized, breathable gas to patient 12.
Sensor 24 connected to flexible tube 14 may monitor treatment and/or event data such as, for example, presence and/or absence of apneas and/or hypopneas, patient response, etc. This information may be sent from sensor 24 to processor 20. Processor 20 may use this information, for example, to adjust the supply of pressurized, breathable gas, log treatment and/or event data, calculate information (e.g. AI, HI, AHI, etc.), etc. Commercial products corresponding to these and similar techniques include CPAP with expiratory pressure relief (EPR), AutoSet, VPAP (with and without a rate setting, which relates to the adjustable respiratory rate or breaths per minute setting), and ADAPT SV, all of which are available from ResMed. More particularly, bilevel devices such as VPAP III available from ResMed and BiPAP REMstar Pro available from Respironics, deliver a higher pressure during inspiration (IPAP), and a lower pressure during expiration (EPAP). Providing different pressures in synchrony with a patients breathing patterns are thought to provide increased comfort.
Positive airway pressure devices (e.g. those functioning in CPAP mode) can treat basic obstructive apneas and offset some hypopneas. Indeed, some commercial devices may offset some hypopneas during the course of apnea treatment. For example, ResMed's AutoSet algorithm provides a minimum set pressure (for example approximately 4 cm H2O) and then detects respiratory events such as apneas, snoring, and flow limitation or flattening, etc. In response to detecting such an event, the pressure is increased at a predetermined rate depending upon the magnitude and type of event that has occurred. When events are not detected, the pressure slowly decreases at a set rate until it reaches the minimum set pressure level. Thus, the pressure is controlled based on respiratory events. This algorithm continuously provides the lowest possible pressure to prevent respiratory events. It will be appreciated that the pressure may change with each patient, based on, for example, body weight, sleep position, sleep stage, drug/alcohol intake, etc. Unlike ResMed's AutoSet algorithm, the Respironics AutoSet algorithm uses the Pcrit model to continuously calculate and adjust the supply of pressurized breathing gas to deliver the gas at Pcrit. Pcrit, or critical closing pressure, is one measure of upper airway collapsibility. Pcrit is based on the idea of modeling the upper airway as a simple collapsible tube. According to this model, Pcrit is the level of nasal pressure below which the upper airway collapses. One advantage of this model is that it gives a global measure of upper airway collapsibility that includes both the structural and neuromuscular factors that determine upper airway collapsibility. It will be appreciated that Pcrit may change with each patient, based on, for example, body weight, sleep position, sleep stage, drug/alcohol intake, etc. Although such AutoSet devices may detect hypopneas, they do not currently adjust the pressure in response to hypopnea events. Furthermore, because a hypopnea is difficult if not impossible to detect unless it is above the Pcrit level, it is similarly important to treat hypopneas above the Pcrit level.
Patients suffering from SDB may experience a combination of apneas and hypopneas. The timing of these conditions and events may vary with, for example, sleep stage, sleep position, weight loss/gain, intake of alcohol and/or medications that may depress the respiratory drive of the patient, etc. Some or all of these variables may be accounted for by enabling a device to measure and deliver varying levels of positive airway pressure to the patient. Although these techniques may reduce patients' apneic events, such treatment techniques have little effect in treating hypopneas. For example, this is because hypopneas tend only to respond to pressures above Pcrit, and some PAP devices only provide pressure levels up to Pcrit.
Thus, it will be appreciated that a need has developed in the art to overcome one or more of these and other disadvantages.